It is known to produce alcohols by hydrogenation, or more properly hydrogenolysis, of monoesters of carboxylic acids. The reaction can be summarised as follows: ##STR1## In the book "Catalytic Hydrogenation" by M. Freifelder, published by John Wiley and Sons Inc. (1978), it is stated at page 129 et seq. that the catalyst of choice is barium promoted copper chromite. In "Organic Reactions", Vol. 8, published by John Wiley and Sons Inc. (1954), Chapter I is by Homer Adkins and is entitled "Catalytic Hydrogenation of Esters to Alcohols". It is suggested that a "copper chromite" catalyst is more correctly described as an approximately equimolar combination of cupric oxide and cupric chromite, i.e. CuO,CuCr.sub.2 O.sub.4. Reference may also be had to Kirk-Othmer's "Encyclopedia of Chemical Technology" (Third Edition), Volume 1, page 733 to 739.
Patent specifications describing hydrogenation of esters to yield alcohols include U.S. Pat. No. 2,040,944, U.S. Pat. No. 2,079,414, U.S. Pat. No. 2,091,800, and FR-A-1276722.
It is known to effect hydrogenation of certain esters and diesters in the vapour phase. For example it has been proposed to use a reduced copper oxide/zinc oxide catalyst for effecting hydrogenation of esters in the vapour phase. In this connection attention is directed to GB-B-2116552. Also of relevance is WO-A-90/8121.
It is further known to produce diols, such as butane-1,4-diol, by catalytic hydrogenation of esters of dicarboxylic acids, such as a dimethyl or diethyl ester of maleic acid, fumaric acid, succinic acid, or a mixture of two or more thereof. Such processes are described, for example, in GB-A-1454440, GB-A-1464263, DE-A-2719867, U.S. Pat. No. 4,032,458, and U.S. Pat. No. 4,172,961.
Production of butane-1,4-diol by vapour phase hydrogenation of a diester, typically a dialkyl ester, of a C.sub.4 dicarboxylic acid selected from maleic acid, fumaric acid, succinic acid, and a mixture of two or more thereof has been proposed. In such a process the diester is conveniently a di-(C.sub.1-4 -alkyl) ester, such as dimethyl or diethyl maleate, fumarate, or succinate. A further description of such a process can be found in U.S. Pat. No. 4,584,419, EP-A 0143634, WO-A-86/03189, WO-A-86/07358, and WO-A-88/00937.
Another commercially important diol is 1,4-cyclohexanedimethanol. This compound is used to prepare highly polymeric linear condensation polymers by reaction with terephthalic acid and is useful as an intermediate in the preparation of certain polyester and polyester amides. The use of 1,4-cyclohexanedimethanol for such purposes is disclosed in, for example, U.S. Pat. No. 2,901,466.
One method for preparing 1,4-cyclohexanedimethanol (hexahydroterephthalyl alcohol) involves the hydrogenation of diethyl 1,4-cyclohexanedicarboxylate (diethyl hexahydroterephthalate) in a slurry phase reactor in the presence of a copper chromite catalyst at a pressure of 3000 psia (about 206.84 bar) and a temperature of 255.degree. C., as is described in Example 3 of U.S. Pat. No. 2,105,664. The yield is said to be 77.5%.
The hydrogenation of dimethyl 1,4-cyclohexanedicarboxylate (DMCD) to 1,4-cyclohexanedimethanol (CHDM) is shown below in equation (2): ##STR2##
Several processes for the production of 1,4-cyclohexanedimethanol and related alcohols have been published since the issuance of U.S. Pat. No. 2,105,664 in 1938. These processes have, almost exclusively, focused on methods for performing the above hydrogenation reaction in the liquid phase. Advances have been reported in the general areas of cis-/trans- isomer selectivity (U.S. Pat. No. 2,917,549, GB-A-988316 and U.S. Pat. No. 4,999,090), catalyst type (GB-A-988316 and U.S. Pat. No. 3,334,149) and conditions of plant operation (U.S. Pat. No. 5030771).
One problem associated with processes for production of a hydroxylic compound selected from alcohols and diols by hydrogenation of a corresponding hydrogenatable material selected from monoesters of carboxylic acids, monoesters of dicarboxylic acids, diesters of dicarboxylic acids, aldehydes, olefinically unsaturated aldehydes, and mixtures of two or more thereof is the synthesis of unwanted by-products which result, for example, from reactions between one or more of the products with themselves or with the starting material or with an intermediate product. Thus methanol is a product, and dimethyl ether is a by-product, of the hydrogenation of methyl caprate or methyl oleate, and are also to be found among the reaction products when dimethyl maleate is subjected to vapour phase hydrogenation. Further types of by-product observed in hydrogenation of alkyl esters of fatty acids are the alkanes corresponding to the desired alcohols and the ethers formed between the alcohol derived from the acid moiety of the ester and the alkyl alcohol derived from the alkyl moiety of the ester. Particularly when hydrogenating alkyl esters of fatty acid mixtures, e.g. methyl esters of mixed C.sub.8 to C.sub.18 fatty acids, such byproduct alkanes and methyl ethers have boiling points which are close to those of the desired product alcohols. Hence they cannot be readily separated therefrom by conventional methods such as distillation. Moreover the presence of such byproducts in the mixed alcohol products has deleterious effects on the downstream uses thereof, e.g. in detergent production.
In vapour phase hydrogenation of n-butyraldehyde possible by-products include n-butyl butyrate and di-n-butyl ether. Although the ester by-product, n-butyl butyrate, can be hydrogenated to useful product, i.e. n-butanol, di-n-butyl ether is not readily susceptible to hydrogenation and hence is an unwanted by-product. When hydrogenating 2-ethylhex-2-enal potential by-products are 2-ethylhexyl 2-ethylhexanoate and di-(2-ethylhexyl) ether, of which the latter is the more undesirable by-product.
By-product formation is also a problem in the production of 1,4-cyclohexanedimethanol. It can be postulated that the observed by-products are formed by reactions involving feed impurities, methanol contained in a recycle hydrogen stream, and the hydrogenation reaction starting material and product themselves. Hence, for example, GB-A-988316 discusses the minimisation of such by-product make by operating the hydrogenation reaction under "relatively mild conditions" (page 2, lines 55 to 79 of GB-A-988316). By the term "relatively mild conditions" is meant a temperature of at least 200.degree. C., preferably between 240.degree. C. and 300.degree. C. and a pressure of 200 to 300 atmospheres (202.65 bar to 303.98 bar), according to page 2, lines 26 to 32 of GB-A-988316. The problem of unwanted by-products is mentioned in the Examples of U.S. Pat. No. 3,334,149 to the extent that certain by-products are included in the tables of results therein. However, there is no discussion in U.S. Pat. No. 3,334,149 as to how such by-product make could be minimised.
One of the major by-products formed during a hydrogenation reaction producing cyclohexanedimethanol according to equation (2) is the product of the reaction between cyclohexanedimethanol itself and its co-product methanol. If the cyclohexanedimethanol is the 1,4-isomer then the product of this reaction is the monomethyl ether, formula: 1-hydroxymethyl-4-methoxymethyl-cyclohexane, which has the ##STR3##
Another by-product which may be formed during production of 1,4-cyclohexanedimethanol is the di-methyl ether, di-(4-hydroxy-methylcyclohexylmethyl) ether, having the formula: ##STR4##
Prior to distillation to effect final purification further by-products can be identified in the crude 1,4-cyclohexanedimethanol product in minor amounts.
The formation of such by-products in the hydrogenation zone results in a loss of the valuable cyclohexanedimethanol product and increases methanol consumption. The presence of by-products in the final cyclohexanedimethanol product requires distillation steps to remove them. The presence of the mono-ether, 1-hydroxy-methyl-4-methoxymethylcyclohexane, in 1,4-cyclohexanedimethanol destined for use in the manufacture of high molecular weight polyesters is desirably avoided since the mono-ether functions as a chain terminator which can result in the formation of an inferior polyester.
In the production of butane-1,4-diol by hydrogenation of a dialkyl maleate typical by-products include gamma-butyrolactone, the corresponding dialkyl succinate, and tetrahydrofuran. Whilst it is possible to hydrogenate both the dialkyl succinate and gamma-butylrolactone to butane-1,4-diol, it is not possible readily to convert tetrahydrofuran to butane-1,4-diol. In some circumstances tetrahydrofuran is itself a valuable product. However, when the primary aim is the production of butane-1,4-diol, tetrahydrofuran can be regarded as an undesirable by-product which serves only to reduce the available yield of butane-1,4-diol. Production of gamma-butyrolactone or dialkyl succinate as a by-product, on the other hand, need not be a disadvantage since either or both of these materials can be recycled to the hydrogenation zone for conversion to the desired product, i.e. butane-1,4-diol.
It is recognised that many of the physical properties required for proper functioning of a catalyst, such as pore diffusion coefficients, are determined by catalyst morphology. Whilst it is not possible to give a complete description of the morphology of a catalyst pellet an approximate definition can be given by reference to average properties such as specific surface area (i.e. total accessible area), specific porosity (i.e. total accessible pore volume), pore size distribution (i.e. distribution of pore volume as a function of pore radius), mean pore radius, and particle size distribution.
In the book "Catalyst Handbook", edited by Martyn V. Twigg, published by Wolfe Publishing Ltd., Second Edition (1989), pores are classified, for example, as macropores (&gt;30-35 nm), micropores (&lt;2 nm), or mesopores (intermediate size). However, these ranges of pore sizes are arbitrary and any other convenient ranges can be used instead to define pore size types.
Whilst it is recognised that the average properties of a catalyst can be used to describe the morphology of a catalyst pellet, it is not possible to predict what effect on the catalyst performance a change in any one of the properties used to describe the morphology will have. Thus it is not possible to predict what effect changes in some or all of the average properties of a catalyst will have on the performance of that catalyst, in particular upon the rate of formation of by-products.